1. Field of the Invention
The present invention relates to a semiconductor optical device with a buried heterostructure (BH) useful for optical communication and to the fabrication method of the same. More particularly, the present invention relates to a buried structure semiconductor optical device structured such that a Ru-doped semi-insulating layer is inserted between a mesa stripe and a burying layer.
2. Description of the Related Art
As the burying layer of a semiconductor optical device such as a semiconductor laser diode or an optical modulator and the like, a current blocking layer formed by a pn buried structure and a current blocking layer formed by a semi-insulating layer are known. According to these current blocking layers, for example, currents can be concentrated on a light-emitting region in a semiconductor laser diode.
Since parasitic capacitance of the current blocking layer formed by the pn buried structure is larger than that of the current blocking layer formed by the semi-insulating layer, it is difficult to realize high-speed operation for the devices with the pn buried structure.
As for a current blocking layer using Fe-doped indium phosphide (InP) as a semi-insulating film, when a Zn-doped InP layer is used as a p type cladding layer, the conductivity of the current blocking layer near the mesa stripe is changed to p type due to inter-diffusion between Fe in the current blocking layer and Zn in the p type cladding layer, so that there occurs a problem in that resistivity of the current blocking layer becomes low. As a result, leakage current and junction capacitance increase. These problems also cause degradation of device characteristics.
That is, inter-diffusion between iron (Fe) and zinc (Zn) occurs when a Fe doped burying layer is placed adjacent to a Zn-doped cladding layer and/or a Zn-doped contact layer. The inter-diffusion causes degradation of device characteristics, especially, modulation characteristics. In addition, Zn atoms moved in an interstitial site due to inter-diffusion diffuse not only to the burying layer but also to an active layer adjacent to the Zn-doped cladding layer (in the case of the semiconductor laser diode), or to a photoabsorption layer adjacent to the Zn-doped cladding layer (in the case of the optical modulator). Thus, there is also a problem in that light emitting efficiency of the active layer is lowered, or the extinction characteristic of the photoabsorption layer is degraded.
Conventionally, there are the following technologies to solve these problems. Japanese laid-open patent application No. 10-22579 discloses a semiconductor laser diode having nondoped InAlAs as the burying layer. That is, since Fe is not doped in the burying layer, inter-diffusion between Fe and the p type dopant does not occur, so that degradation of characteristics due to inter-diffusion does not occur. However, since InAlAs is nondoped, there is a problem in that resistivity of InAlAs is low.
In addition, Japanese laid-open patent application No. 9-214045 discloses that a Fe diffusion preventing layer is inserted between a Zn-doped cladding layer and a Fe-doped InP burying layer. That is, as shown in FIG. 1, the Fe diffusion preventing layer 16 is inserted between the Fe-doped InP burying layer 17 and the mesa stripe which is formed by a buffer layer 12, an active layer 13, a cladding layer 14 and a contact layer 15. In the Japanese laid-open patent application No. 9-214045, as a specific example of the Fe diffusion preventing layer 16, an n-InP layer and a Fe-doped InP layer are disclosed, in which vacancy concentration of the Fe-doped InP layer is equal to or more than 5.0×1014 cm−3. 
However, in order to grow the Fe-doped InP layer of which vacancy concentration is equal to or more than 5.0×1014 cm−3 as the Fe diffusion preventing layer 16, it is necessary to use a higher growth temperature (660° C.) than that used for growing the usual Fe-doped InP layer. Thus, thermal degradation may occur on the sides of the mesa stripe during growth.
In addition, although diffusion of Fe can be prevented by inserting an n-InP layer as the Fe diffusion preventing layer 16, there is a problem in that leakage currents increase since resistivity of the n-type InP layer between the cladding layer and the burying layer is low.
In addition, Japanese laid-open patent application No. 61-290790 discloses that a burying layer of Fe-doped InAlAs is formed by liquid phase epitaxy. Also in this case, as mentioned above, there is a problem in that Zn—Fe inter-diffusion occurs between the Zn-doped cladding layer and the Fe-doped InAlAs burying layer.
Recently, it was found that Ru-doped semi-insulating layer rarely causes inter-diffusion between Ru and Zn. Thus, a buried structure laser diode using a Ru-doped InP layer which is a semi-insulating film as the current blocking layer is proposed in A. van Geelen et al., Appl. Physics Letters 73, No 26 pp 3878–3880 (1998), and A. van Geelen et al., 11th International Conference on Indium Phosphide and Related materials TuB1-2 (1999) for example. FIG. 2 shows the configuration.
However, as for the Ru-doped InP burying layer proposed in the above-mentioned documents, a precipitate of Ru—P is apt to occur. Thus, there is a problem in that it becomes difficult that Ru effectively acts as the semi-insulating dopant of InP.
Therefore, in order to suppress occurrence of the Ru-P precipitate, it is necessary to grow the semi-insulating layer under very restricted conditions such as under lowered growth pressure, or lowering the supplying amount of phosphine (PH3), which is the source material for P, or under low growth temperature of about 580° C. or the like.
In patent DE19747996C1, when the number of hydrogen groups of the group V precursor is equal to or less than 2, the growth temperature of the Ru-doped compound semiconductor can be lower than that for PH3 or AsH3 with 3 hydrogen groups, which is mainly used. Since the decrease in the number of hydrogen groups reduces the decomposition temperature of the group V precursor, the growth temperature can be lowered, so that occurrence of the precipitate with Ru can be suppressed.
When growing the semi-insulating layer at the low growth temperature as mentioned above, there occurs a problem in that a defect such as hillock is apt to occur on the surface of the burying layer.
In addition, as for the Ru-doped InP, the surface of the crystal becomes very sensitive, and poor crystal habit easily occurs. Thus, depending on the condition of the surface layer after performing RIE (Reactive Ion Etching) or wet etching, a void may occur in the Ru-doped InP burying layer 30 as shown in FIG. 3. In FIG. 3, the reference numeral 10 indicates an n-InP substrate, 20 indicates the semiconductor stacked body, 21a indicates an n-InP cladding layer, 22a indicates an active region formed by a MQW active layer or MQW photoabsorption layer, 23a indicates a p-InP cladding layer, 24a indicates a p-InGaAsP contact layer, 25a indicates a p-InGaAs contact layer, 31a indicates a void in the side wall of the mesa stripe, and 31b indicates a void in the side wall including InAlAs.
In addition, when burying a device with the active region formed by an InAlAs—InGaAlAs multiple quantum well layer that acts as the active layer or the photoabsorption layer, a void easily occurs on the side wall of the active region. Thus, there is a problem of reliability, reproducibility and the like. In addition, it is difficult to change a physical constant such as the lattice constant and the index of refraction for the InP layer and the like.
There is a method of mass transport as a method for burying the side surface of the active region as disclosed in Japanese laid-open patent application No. 8-250806, for example.
Processing damage on the side surface of the active region due to formation of the mesa stripe is removed by using wet etching. After that, the device is loaded in a growth reactor. When the growth temperature rises, a part of the cladding layer is dissolved, is moved to the side surface of the active region and recrystallized. As a result, the side surface of the active region is buried by a material for the cladding region.
According to this method, surface damage of the processed layer due to dry etching can be removed, and thermal damage due to the rise in temperature can be prevented.
However, by this mass transport method, since the part of the material of the cladding region is dissolved, moved to the side surface of the active region and recrystallized for performing the burying process, impurity doped in the cladding layer is moved with the material and included in the burying layer that is recrystallized. Therefore, the area of junction region may increase and leakage currents may increase.
In addition, the impurity included in the material of the dissolved cladding layer diffuses to the active region adjacent to the burying layer, and the diffusion may cause deterioration of light emitting characteristics of the active layer or extinction characteristics of the photoabsorption layer. Especially, when Zn, which is a p-type impurity, is included in the region formed by the mass transport, the p-type burying layer and the region formed by mass transport are connected in the case of pn-junction buried structure, or, it causes inter-diffusion between Fe and Zn, and leakage currents and junction capacitance increase when using a Fe-doped semi-insulating layer as the burying layer.
As mentioned above, a Fe-doped semiconductor crystal is used for the semi-insulating buried heterostructure (SIBH). However, there is a problem in that inter-diffusion between Fe in the burying layer and Zn, which is a dopant of the p-cladding layer and p-contact layer, occurs at the interface of the burying layer and the Zn-doped layer. As a result, Zn atoms diffuse to the burying layer, which causes device characteristics degradation, especially, modulation characteristics degradation.
In addition, in a conventional technology using a Ru-doped InP burying layer such as A. van Geelen et al., Appl. Physics Letters 73, No 26 pp 3878–3880 (1998), bis(η5–2,4-dimethylpentadienyl ruthenium(II)) is used as a source material gas of Ru, and an InP crystal in which Ru is doped to 4×1018 cm3 is grown by using a metalorganic vapor phase epitaxy method. The mass transport is not used.
In a conventional technology disclosed in A. van Geelen et al., 11th International Conference on Indium Phosphide and Related materials TuB1-2 (1999), fabrication of a semiconductor laser diode is disclosed in which a Ru-doped semi-insulating InP layer and an n-InP hole blocking layer formed on the Ru-doped InP layer are used as the burying layer. Also in this example, growth of the burying layer is performed by an epitaxial growth method using the MOVPE method, and mass transport is not used.
That is, when using mass transport in the conventional technology, there are problems in that the region formed by mass transport becomes p type, junction capacitance increases, and leakage current paths occur. Therefore, performance of the device degrades, and there is a problem in that fabrication yield is low. In addition, Ru doping is performed by the epitaxial growth method and mass transport is not used.